Many women in the U.S. desire to lose weight during the postpartum period and often turn to caloric restriction or exercise to help accomplish this goal. However, it is unknown to what extent energy balance can be altered before lactation is compromised. The goal of the proposed research is to evaluate the effect of short-term caloric restriction or exercise with caloric restriction on lactation performance. Women who are exclusively breastfeeding at 8-12 wk postpartum will be randomly assigned to one of 3 groups (N=25/group) for a period of 11 d: a) control, given 100% of energy needs; b) caloric restriction to 65% of energy needs; and c) exercise plus caloric restriction to create a similar caloric deficit as in group b. Measurements of baseline energy intake and expenditure will be used to determine initial energy needs and will serve as the basis for the diets furnished and exercise levels prescribed during the intervention. Data on milk volume and composition, plasma prolactin, maternal body composition and infant weight will be collected prior to and during the 11-d intervention period. Milk volume will be determined by 3-d test-weighing using an electronic balance. Milk samples will be collected by alternate breast expression over 24 h and analyzed for fat, lactose, protein, energy and fatty acid concentrations. Plasma prolactin levels will be determined by radioimmunoassay from blood samples collected by indwelling catheter before, during and after a breastfeeding episode. Maternal body composition will be assessed by hydrostatic weighing and deuterium dilution in respiratory water. Repeated measures analysis of variance will be used to examine the impact of the intervention. The results of this study will be useful for determining a safe minimal caloric intake during lactation for U.S. women.